The effect of recombinant human granulocyte-macrophage colony-stimulating growth factor (rHuGM-CSF) treatement on in vitro interferon (IFN) and tumor necrosis factor (TNF) production in peripheral blood cells of 46 patients with acute myelogenous leukemia (AML) was examined. GM-CSF significant enhanced virus-induced IFN- production in blood cells (containing 70% of blasts) of 28 patients with M4-M5 AML according to the French-American-British (FAB) classification and also phytohemagglutinin (PHA)-induced IFN- production in blood cells (containing 68% of blasts) of 18 patients with AML M0-M3 type. In control blood cells (25 healthy persons) GM-CSF enhanced PHA-induced IFN- but did not influence IFN- production. In the presence of GM-CSF, TNF- titers induced with lipopolysaccharide were also higher in control blood cells but not in cells of patients with M0-M3 or M4-M5 type of AML. The significance of GM-CSF-enhanced IFN- and IFN- production in antimicrobial and antileukemic immune reactions which can develop during GM-CSF therapy is discussed.